1. Field of the Invention
The present invention relates to an optical projection system and, more particularly, to an optical system of unit magnification for transmitting an erect real image of an object from an object plane onto an image plane.
2. Description of the Prior Art
Examples of such an optical projection system have been found in various machines, for example, in copying machines of the kind wherein a copy of an original is produced during a transfer step which causes the transfer of a developed image from a photosensitive plate or drum to a copy paper. The optical projection system employed in the early copying-machines of the kind consists of a single lens device which projects the entire image of an object at a time on the image plane. A disadvantage of that system is that in order to obtain good images free from the aberrations it must have a relatively long focal length and therefore requires a relatively long track length between the object and the image planes, resulting in the increase in size of the copying-machines.
With increasing demand for compact copying-machines, there have been proposed various optical projection systems of the kind wherein a plurality of lenses with a short focal length are arranged in one or more rows to scan strip-shaped portions of an object in the object plane and to project images of the scanned strip-shaped image elements onto the successive image plane where a complete image of the scanned object is formed.
For example, U.S. Pat. Nos. 3,584,950 and 3,584,952 (corresponding to Japanese patent publication No. Sho49-8893) discloses an optical projection system comprising a plurality of optical imaging devices, each of which comprises three separated lens elements, i.e., the first lens element arranged nearer to the object plane to form an inverted real image on an intermediate image plane, the second lens element arranged in the intermediate image plane to prevent the imaging device from experiencing any loss of image light rays, and the third lens element arranged nearer to the image plane to transmit and invert the intermediate image onto the image plane. In such a system, the image rays projected by the adjoining imaging devices are slightly overlapped by one another to reduce the uneveness of exposure and, the size of an elemental area to be scanned at one time by each imaging device is defined by an aperture plate adjacent to the second lens element.
Optical projection systems similar to those of the above U.S. patents are disclosed in DT-OS No. 29 05 740 and U.S. Pat. No. 3,592,542.
Also, U.S. Pat. No. 4,215,933 discloses a projection system comprising a plurality of unsymmetrical lenses each comprising three lens elements and a lens-aperture plate arranged in an intermediate plane.
These projection systems have advantages in that they produce compact copying-machines because of their very short focal length and in that they can be constructed by plural lenticular bars each comprising a plurality of lenticular elements. It is, however, difficult to obtain good quality of copies with these projection systems. Because, the lens-aperture plate is located in a position adjacent to the second lens element or in the intermediate image plane where all the image light rays are gathered, so that the light exposure on the image plane is greatly affected by an error in measurement in making the lenses, particularly, by an error in size and disposition of the aperture plate. The deviation of exposure due to that error reaches at least several percent in spite of the fact that the allowable deviation of exposure in the copying machines is less than a few percent. As a result, the ultimate copy obtained exhibits bright and dark streaky images extending in the direction parallel to the scanning direction, resulting in a considerable decrease in the quality of the copy.
It is therefore a general object of the present invention to provide an improved optical projection system with a very short focal length that avoids the above drawbacks and minimizes the uneveness of exposure.
Another object of the present invention is to provide an improved optical projection system of unit magnification that has a construction format of great advantage to manufacturing.
According to the present invention, there is provided an optical projection system of unit magnification for transmitting a real erect image of an object from an object plane to an image plane, that comprises a plurality of lenses having parallel optical axes and being arranged in one or more rows, each of said lenses comprising three lens elements arranged along a common optical path extending between the object and image planes, the first lens element nearer to the object plane being adapted to form an inverted intermediate image upon an intermediate image plane, the second lens element being arranged at or near the intermediate plane to prevent the lens from experiencing the loss of light rays, the third element nearer to the image plane being adapted to transmit and invert the intermediate image on the image plane, characterized in that each of said lenses includes two aperture masks which subtend light rays from the axial and off-axis object points in the object plane, one of said aperture masks being arranged between the first and second lens elements and at a position closer to the first lens element and farther from the second lens element, the other mask being arranged between the second and third lens elements and at a position closer to the third lens element and farther from the second lens element.
In a preferred embodiment of the present invention, the above lenses are so designed that they have a magnification of an intermediate image ranging from -0.25.times. to -0.6.times..
According to another feature of the present invention, there is provided an optical projection system of unit magnification for transmitting an erect real image from an object plane to an image plane, comprising a plurality of lenses having parallel optical axes and being arranged in one or more rows, each of said lenses comprising four lens elements arranged along a common optical path extending between the object and image planes, said lens elements being classified into two groups which are symmetric with respect to an intermediate image plane, the first groups consisting of two lens elements on the side of the object plane and forming a real inverted intermediate image of the object on the intermediate plane, the second group consisting of two lens elements on the side of the image plane and transmitting and inverting the intermediate image on the image plane, characterized in that each of said lenses includes at least two aperture masks which subtend light rays from the axial and off-axis object points in the object plane, one of said aperture masks being arranged between two lens elements of said first group, the other being arranged between two lens elements of said second group.
In one preferred embodiment of the above projection system, the above lenses comprising four lens elements are so designed that they have a magnification of an intermediate image ranging from -0.3.times. to -0.7.times..
According to a further feature of the present invention there is provided a projection system of unit magnification for transmitting an erect real image from an object plane to an image plane, comprising a plurality of lenses having parallel optical axes and being arranged in one or more rows, each of said lenses comprising four lens elements arranged along a common optical path extending between the object and the image planes, said lens elements being classified into two groups which are symmetric with respect to an intermediate plane, the first group consisting of two lens elements on the side the object plane and forming a real inverted image of the object on the intermediate plane, the second group consisting of two lens elements on the side of the intermediate image plane and transmitting and inverting the intermediate image on the image plane, characterized in that each of said lenses includes two aperture masks which subtend light rays from the axial and off-axis object points, one of said aperture masks being arranged in front of the first group, the other mask being arranged at the back of the second group.
In one preferred embodiment of the above projection system, the above lenses comprising four lens elements are so designed that they have a magnification of an intermediate image ranging from -0.35.times. to -0.7.times..
In the above three types of the projection system of the present invention, the lens element may be made of a synthetic plastic material or conventional optical glass. In a preferred embodiment, the lenses are constructed by plural lenticular sheets each including a plurality of lenticular elements arranged in one or more rows. The lens elements may be so designed that they have refracting surfaces, of which all the radii of curvature are numerically equal.
In another preferred embodiment of the present invention, the above lenses are arranged in two or more parallel rows so that the lenses in one row are shifted by a distance equal to one-half of the distance between the neighboring optical axes of any two of the lenses in the neighboring row.
These and other objects, features and advantages of the present invention will be further apparent from the following description taken in conjunction with examples and the accompanying drawings.